Process of chemical nickel plating of amphoteric elements and their alloys



March 15,

ROCESS OF CHEMICA w. G. LEE ETAL 8,757

L NI

2,92 L PLATING 0F AMPHOTERIC LOYS ELEMENTS THEIR AL Filed NOV. 2-7, 195777 Alloy Body INVENTOR. Warren 6'. Lee BY Emil/an Browar 4 JWZGW M2,928,757 rnocnss or CHEMICAL NICKEL L TING 6F AMPHOTERIC ELEMENTS ANDTHEIR ALLOYS Warren G. Lee, East Chicago, and Emilian Browar, Gary, Ind,assignors to General American Transportation Corporation, Chicago, 11].,a corpo'ratioii of New York Application November 27, 1957, Seriztl ldo.699,373 9 Claims. cl. 117-50) The present invention relates to chemicalmeter plating of the amphoteric metals of group Vl- B of the periodicsystems of elements, and their alloys, and to processes of effectingsuch plating with aqueous chemical nickel plating baths of the nickelcation=hypophosphite anion type.

Heretofor'e the chemical nickel plating of workpieces having surfacesformed essentially of the amphoteric metals of group IV B and theiralloys, has been of little practical utility, as such coatings as havebeen produced thereon readily flake-off, since the coatings are notintimately bonded thereto. This statement is particularly applicable tothe important amphoteric element titanium, although it is applicable tothe other elements of this group that consists of: titanium, zirconiumand hafnium.

The principal reason for this lack of adhesion between the amphotericmetal surface and the coating is that such a surface is extremelysensitive to hydrolysis and iin' mediately films over upon contact withthe aqueous chemical nickel plating bath; whereby there is a film layerat the interface between the amphoteric metal surface and the coating ofnickel-phosphorus alloy that is inherently produced by a plating bath ofthis type. Of course, this prevents initiation of the plating andintimate bonding between the amphoteric metal surface and the coating,-With the result that the coating exhibits no substantial adhesion to thesurface carrying the same;

Accordingly, it is a general object of the invention to provide aprocess of chemical nickel plating of a workpiece having an amphotericmetal surface, wherein the coating produced is intimately bonded to thesurface and exhibits great adhesion thereto.

Another object of the invention is to provide a process of preparing thearnphoteric metal surface of a workpiece for plating with chemicalnickel plating bath so that the subsequently produced coating carriedthereby is intimately bonded to the prepared surface.

A further object of the invention is to provide a process of thecharacter noted, that is expressly adapted for use in the chemicalnickel plating of titanium and its alloys.

A further object of the invention is to provide a process of thecharacter noted that involves a heat-treatment step following the nickelplating of the amphoteric metal surface, whereby a diffusion reaction isproduced between the arnphoteric metal surface and the nickel-phosphorusalloy coating, whereby the interface is alloyed to produce an intimatebond therebetween.

Further features of the invention pertain to the particular arrangementof the steps of the method, whereby the above-outlined and additionaloperating features thereof are attained.

The invention, both as toits organization and method of operation,together with further'objects and advantages thereof, will best beunderstood by referenceto the fol= lowing specification taken inconnection with the accorri- CPI ice

Figure 2 is an enlarged fragmentary view, partially in section, of aportion of the compressor rotor, shown in Fig. 1', illustrating themanner in which one of the blades is detachably secured to theassociated support and involving' a threaded connection therebet'ween;

Figure 3 is a greatly enlarged sectional View of the threaded connectionor joint between the root of the blade and the support, illustrating theprotective coatings respectively carried by these two parts andpreventing galling the'rebetween; and

Figure t is a fragmentary vertical sectional view of another machinethat is made in accordance with the process of the present invention:

Referring now to the drawing, there is illustrated in Fig. 1 afragmentary portion of a compressor rotor 10" that may comprise a partof the air compressor that cooperates With an associated gasi'l'l'rb'iri if! a jet engine, or the like. More particularly, the rotor10 comprises a support 11 of substantially drum-like configuration, anda plurality of annular rows of blades 12 disposed about the outersurface thereof; The rotor 1t) is housed in an associated casingorstator,- not shown, and mounted forrotation at high speed therein sothat the annular rows of blades' 12 cooperate with other annular rows ofblades carried internally of the stator, so as to provide a multi-vstage centrifugal air compressor. In the rotor 10, the support 11 andeach of the blades 12 is formed of a suit able titanium alloy,- and eachof the blades 12 is detach ably secured to the support 11 to accommodatereplace- As best shown in Fig. 2, each of the blades 12 terminates atthe base thereof in a threaded root 12a that is normally arranged inthreaded relation with a cooperatin threaded spar ng 11a provided in thesupport 11'; and as illustiated in Fig. 3, the threads carried by theroot 12d are designated 12f and the threads carried by the wall of thesupport 11' about the opening 11a therein are designated l lt. Also, asshown in Fig; 3, the threads 12! and the root I ZQa'Q'and the adjacentshoulder provided on the base of the blade 12 carry a unitary coating12c, and the threads 11! andthe surface of the support 11 adja'cerit tothe opening 11 therein carry a unitary coating 11; wherebythe coatings12cland are in direct fricv herently produced by chemical nickel platingfrom an aqueous" chemical n'iek'el plating bath of thenickelcationhypophosp'hite anion type; and such alloy essentiallycomprises 89% to 97% nick eland 3% to 11% phosphorus, by weight.

The arrangement of the coatings 11c and 12c in direct engagement witheach other is Very advantageous as these coatings'preve'nt galling thatwould otherwise take place between the parts if and 12 formedessentially of titanium, with the resulting freezing of these partstogether so that subsequently itwouldb'e' impossible to remove the bladelz'fro'ni the support 11, a's'is the usual case where titanium" alloyparts are arran'ged'in pressure engagement with each other. However, thecoatings 11c and do notexhibit g'a'llingttherebetween, whereby thetitanium alloy: parts' li and li may. be readily. disassembled aii'dreassembled at will, without damage thereto.- I

This anti-galling characteristic thus imparted to the titanium alloypartsi1 audit. by the protective nickel- Percent Al 4 Mg 4 Ti RemainderThese titanium alloys normally contain titanium in the general range 90%to 95% and other alloying elements in the range 5% to Another typicaltitanium alloy of this character contains the following principalcomponents:

Al 3.18 Cr 4.68 Fe 0.22 Ti Remainder Still another typical titaniumalloy of this character contains the following principal components:

Cr 2.84 Fe 1.43 Ti Remainder A further typical titanium alloy of thischaracter contains the following principal components:

Mn Ti 7.8 Remainder The composition of the titanium alloy is in no waycritical with respect to the present process. since any such alloycontaining in excess of 90% titanium exhibits the previously mentionedamphoteric characteristic rendering it impossible to obtain anintimately bonded nickel-phosphorus alloy coating thereupon, withoutspecial treatment in accordance with the present process.

In the foregoing description of the rotor 10 it was explained that theelements 11 and 12 are provided with the respective coatings 11c and120, and while this is the preferred construction, it will beappreciated that really only one of the coatings 110 or 120 is essentialto prevent the titanium-upon-titanium contact and resulting gallingtherebetween. More particularly, there is no galling of a titanium alloyupon a nickel-phosphorus alloy of the character described.

Referring now to Fig. 4, a fragmentary portion of a machine or device 20is illustrated that comprises a support or plate 21 having an opening210 formed therein into which a shaft or bar 22 is mounted and arrangedfor relative movements with respect thereto, the shaft 22 being mountedfor rotation about its longitudinal axis or longitudinal slidingmovement, or both. The plate 21 and the shaft 22 are formed of suitabletitanium alloys and respectively'carry chemically depositednickel-phosphorus alloy coatings 21c and 22c arranged in engagement witheach other, so as to prevent galling between the elements 21 and 22, aspreviously explained. This bearing arrangement is very advantageous asthe nickelphosphorus coatings 21c and 22c resist wear and may belubricated with any light oil, or the like.

In the foregoing description of the machine 20 it was explained that theelements 21 and 22 are provided with the respective coatings 21c and220, and while this is the preferred construction, it will beappreciated that really only one of the coatings 210 or 220 is essentialto prevent the titanium-upon-titanium contact and resulting gallingtherebetween.

Considering now the process of the present invention, the work-pieceformed essentially of titanium, and nor- 4 mally comprising an alloythereof, as previously noted, is first subjected to a conventional vapordegreasing step in order to effect substantial and ordinary cleaningthereof. Next the work-piece is subjected to alkaline cleaning (such asEnthone #160, or equivalent) to remove therefrom bufiing compounds,etc., that may have been employed in the machining thereof; and normallya hot aqueous solution of the alkaline cleaner is employed at atemperature of about 195 F. throughout a time interval of about 2 to 5minutes. The work-piece is then rinsed in cold water.

Next the work-piece is pickled by immersion thereof for about 10 toseconds in a special pickling solution described hereinafter; and thenthe work-piece is again rinsed with cold water. The two last-mentionedsteps are repeated so that the work-piece is again subjected to-the withcold water for about 10 to 15 seconds.

time, the surface of the work-piece has been conditioned pickling stepfor about 10 seconds and thereafter rinsed At this it essentiallycomprises an aqueous solution of hydrochloric acid, hydrofluoric acidand a mild oxidizing agent, both the hydrochloric acid concentration andthe hydrofluoric acid concentration being high. Specifically, thepickling bath may be composed from 900 cc. :10 cc. hydrochloric acid (18B.) and cc. :2 cc. hydrofluoric acid (70%), together with the mildoxidizing agent, such, for example, as FeCl .6I-I O in the approximaterange 25 to 100 gms. This pickling bath thus comprises about 8mole/liter of hydrochloric acid, about 4% mole/liter of hydrofluoricacid and about 0.1 to 0.4 mole/ liter of ferric chloride. Specifically,in formulating the plating bath, the following proportions arerecommended:

900 cc. HCl solution (18 B.)

100 cc. HF solution (70%) 50 gm. FeCl .6I-l O In preparing the picklingbath the two acid solutions are mixed, and the required amount of ferricchloride is melted in a steam bath and then added to the acid mixtureunder agitation.

While ferric chloride is preferred as the mild oxidizing agent, becauseof the simplicity and economy thereof, other suitable oxidizing agentsmay be employed, such, for example, as the water soluble salts of iron:ferric sulfate, ferric acetate, etc.; and the water soluble salts ofmanganese: manganese chloride, manganese sulfate, manganese acetate,etc.

Apparently, the fundamental modulus of this pickling bath is dependentupon this high concentration of hydrochloric acid in combination withthis high concentration of hydrofluoric acid, since with thiscomposition of the pickling bath there is removed the amphoteric filmthat immediately forms upon the surface of the titanium work-pieceincident to contact thereof with the aqueous chemical nickel platingbath, and as a result of hydrolysis. It is postulated that this filmthat normally forms upon the surface of the titanium work-piece involvesthe equilibrium:

In any case, the film that is present upon the surface of the titaniumwork-piece is effectively removed by this pickling bath, so that thesubsequently applied coating of the nickel-phosphorus alloy in thechemical nickel plating bath is continuous and intimately bondedthereto.

A number of suitable plating baths are available for the presentpurpose,as disclosed in US. Patent No. 2,532,283, granted on December 5, 1950,to Abner Brenner and Grace E. Riddell, in US. Patent No. 2,658,841,granted on November 10, 1953, to Gregoire Gutzeit and Abraham Krieg, andin U.S. Patent No. 2,658,842, granted on November 10, 1953, to GregoireGutzeit and Ernest J. Ramirez, as well as in: the copending applicationof Gregoire Gutzeit, Serial. No, $376,917., filedt August 27., 1953, thecopending application. of, Gregoire Gutzeit', Paul Talmey and: Warren G.Lee, Serial, No. 478, filed December 29, 1954, and in the,copendingapplica tion of Gregoire G-utzeit, Raul, T almey: and Warren,G. Lee, Serial No. 569;8 1 5, filed- March 6,, 195,6, now Batent No.2,822,294, grantedt February 4, 1958.

A chemical nickel plating bath of; this type essentially comprises anaqueous solution of nickel: cations and hypophosphite anions, thenickelcationsbeing derived from nickel sulfate, nickel chloride,nickel;hypophosphite,etc., and the hypophosphite anions being derivedfrom hypophosphorous acid, sodium hypophosphite, potassiumhypophosphite, nickel hypophosphite, etc, Preferably, such a platingsolution comprises an absolute concentration of hypophosphite anionswithin: the range 0.15 to 1.20 moles/liter, a ratio between nickelcations and hypophosphite anions expressed in: molar CQncentrat-ionswithin the range 0.25 to 1,60 and a, in; the; range 4.5V to IL. Theplating bath disclosed. in; the Gutzcit, Talmey and Lee application,Serial NO. 569,815, is; particularly advantageous and comprises, in;addition to the nickel cations and the hypophosphite anions, lacticanions and propionic anions, and having a pH in the acid range 4.4 to5.6; and specifically this; plating bath; comprises an absoluteconcentration of the hypophosphite anions within the range 0.15 to 1.20moles/liter, a ratio. between nickel cations and hypophosphite anionsexpressed in molarconcentrations within the range 0.25 to 1.60, anabsolute concentration of lactic anions. within the range 0.25. to 0.60mole/liter, and an absolute concentration of propionic ions Within therange 0.025 to 0.060 mole/liter. A typical plating bath of this. typehas the following composition:

Nickel ion (as nickel sulfate) ..m.p.l 0.08 Hypophosphite ion (as sodiumhypophosphite)v m.p.l 0.225 Lactic ion (as lactic acid) m.p.l 0.30Propionic ion (as propionic acid) m.p.l (1.03

pH (adjusted with H230 and NaOH) 4.7

In the plating step, the Work-piece is immersed in the chemical nickelplating bath, while the plating bath is at a relatively hightemperature, normally: in the general range 94 C. to 98 C., throughoutan, appropriate time interval depending upon the thickness of the nickelplating required. For example, the particular plating bath set forthabove has a normal platingrateof about 1 mil/hour; and of course, aspreviously noted, the coating that is inherently produced; by; a platingbath of this type essentially comprises anickel-phosphorus alloycontaining about 89% to 97% nickel and about 3%: to 11% phosphorus, byweight. In this step, the work-piece is suspended in the plating bath byan insulating line (Dacron etc.) and out of contact with any metallicpart, so as to avoid any galvanic action, and the resulting surfaceblemish or discontinuity.

Following this chemical plating step, the work-piece is subjected toheat-treatment that merely constitutes elevating the temperature thereofto about 425 C., where by a difiusion reaction takes place at theinterface between the surface of the titanium work-piece and thenickel-phosphorus alloy coating. The diffusion. reaction mentionedoccurs at the temperature 421 C, (8003 F.); whereby there is probablyformed in the interface a ternary alloy of titanium and nickel andphosphorus. This ternary alloy in the interface intimately bonds thenickel-phosphorus alloy coating to the titanium workpiece; and theresulting coating is highly corrosive-resistant, smooth and bright, andprevents the normal galling of titanium-upon-titanium, as previouslyexplained.

In the foregoing description of the present process, the work-piece hasbeen described as being formed essentially of titanium or a suitabletitanium alloy. ,How-

ever, the. nrc esshas een. r e out s cessfully-ere cisely in. the.manner, described When the worlepicde has been r d: f. he. ot r. of h aph ri elem t of. group, IV B including: zirconium alloys and hafni malloys. For example, the heretofore experienced; difficulty in thechemical. nickel plating of the zirconium ailoy. surface of a work-piecehas resided in the circumstance that this amphoteric metal (liketitanium). is subject to the formation, of a film upon the outer surfacethereof due tohydrolysis upon immersion in the aqueous chemical nickelplating solution. However, in accord ance with the present process, thefilm mentioned is re-. moved from the outer surface and the reformationof this film thereupon. isinhibited; whereby upon. contact with theaqueous. chemical nickel plating solution, the plating reaction,takesplace directly upon the, zirconium alloy surface of the work-piece,without the interfering film at the interface. Of course, in, this.example, and following the chemical nickel plating of the zirconiumsurface'of the work-piece, the heat-treatment step at the elevatedtemperature of about 425 C. is, employed, so as probably to produce aternary alloy at the interface comprising; zirconium and nickel andphosphorus.

In view of the foregoing, it is apparent that there has been provided animproved process of chemical nickel plating of work-pieces havingamphoteric metal surfaces, as wellas an improved subcombination in theprocess with respect to the preparation of the surfaces noted for thesubsequent chemical nickel plating of the coatings thereupon. Also theprocess is productive of an improved. article of manufacture andparticularly in an, assembly of two parts formed essentially of titaniumand completely devoid of galling therebetween.

While there has been described what is at present considered tobe thepreferred embodiment of the invention,

it willbe understood that various modifications may be,

made therein, and it is intended to cover in the appended claims allsuch modifications as fall within the true spiritand scope of theinvention,

What is claimed is:

1. The process of plating with nickel; the metal surface of awork-piece, wherein said metal surface essentially comprises anamphoteric element selected from the group consisting of titanium,zirconium and hafnium; said process comprising cleaning saidv metalsurface by contact with a suitable cleaning bath, then conditioning saidmetal surface by'contact with a pickling bath, wherein said picklingbath essentially comprises an aqueous solution of hydrochloric acid andhydrofluoric acid and a. mild oxidizing agent, the absoluteconcentration of hydrochloric acid in said pickling bath being about 8mole/liter, the absolute concentration of hydrofluoric acid in saidpickling bath beingabout 4 /2 mole/liter, then contacting said metalsurface with a chemical plating bath of the nickel cation-hypophosphiteanion type throughout a suflicient time interval to obtain a nickelplating upon said metal surface, and then heating said work-piece to atemperature at least as high as about 425 C. so as to effect a diffusionreaction at the interface between, said nickel plating and said metalsurface.

2. The process of plating with nickel the metal surface of a work-piece,wherein said metal surface essentially comprises an amphoteric elementselected from the group consisting of titanium, zirconium and hafnium;said process comprising cleaning said metal surface by contact with asuitable cleaning bath, then conditioning said metal surface by contactwith a pickling bath, wherein said pickling bath essentially comprisesan aqueous solution of hydrochloric acid and hydrofluoric acid and asoluble salt of iron, the absolute concentration of hydrochloric acid insaid pickling bath being about 8 mole/ liter, the absolute concentrationof hydrofluoric acid in said pickling bath being about 4 /2 mole/liter,then immersing said work-piece in a chemical plating bath of the nickelcation-hypophosphite anion type throughout 7 a sufficient' time intervalto obtain a nickel plating upon said metal surface, and then heatingsaid work-piece to a temperature at least as high as about 425 C. so asto effect a diffusion reaction at the interface between said nickelplating and said metal surface.

3. The process of plating with nickel the metal surface of a Work-piece,wherein said metal surface esssentially comprises a titanium alloy, saidprocess comprising cleaning said metal surface by contact with asuitable cleaning bath, then conditioning said metal by contact with apickling bath, wherein said pickling bath essentially comprises anaqueous solution of hydrochloric acid and hydrofluoric acid and a mildoxidizing agent, the absolute concentration of hydrochloric acid in saidpickling bath being about 8 mole/liter, the absolute concentration ofhydrofluoric acid in said pickling bath being about 4 /2 mole/liter,then contacting said metal surface with a chemical plating'bath of thenickel cation-hypophosphite anion type throughout a suificient timeinterval to obtain a nickel plating upon said metal surface, and thenheating said work-piece to a temperature at least as high as about 425C. so as to effect a diffusion reaction at the interface between saidnickel plating and said metal surface.

4. The process of plating with nickel the metal surface of a work-piece,wherein said metal surface essentially comprises a titanium alloy, saidprocess comprising cleaning said metal surface by contactwith a suitablecleaning bath, then conditioning said metal surface by contact with apickling bath, wherein said pickling bath essentially comprises anaqueoussolution of hydrochloric acid and hydrofluoric acid and a solublesalt of iron, the absolute concentration of hydrochloric acid in saidpickling bath being about 8 mole/liter, the absolute concentration ofhydrofluoric acid in said pickling bath being about 4 /2 mole/liter,then inunersing said workpiece in a chemical plating bath of the nickelcation-hypophosphite anion type throughout a sufficient time interval toobtain a nickel plating upon said metal surface, and then heating saidwork-piece to a temperature at least as high as about 425 C. so as toeffect a diffusion reaction at the interface between said nickel platingand said metal surface.

5. The process of preparing the metal surface of a Work-piece forplating thereon by a .chemical plating bath of the nickelcation-hypophosphite anion type, wherein said metal surface essentiallycomprises an amphoteric element selected from the group consisting oftitanium, zirconium and hafnium; said process comprising cleaning saidmetal surface by contact with a suitable cleaning bath, and thenconditioning said metal surface by contact with a pickling bath, whereinsaid pickling bath essentially comprises an aqueous solution ofhydrochloric acid and hydrofluoric acid and a mild oxidizing agent, theabsolute concentration of hydrochloric acid in said pickling bath beingabout 8 mole/liter andthe absolute concentration of hydrofluoric acid insaid pickling bath being about 4 /2 mole/ liter.

6. The process of preparing the metal surface of a work-piece forplating thereon by a chemical plating bath of the nickelcation-hypophosphite anion type, wherein said metal surface essentiallycomprises an amphoteric element selected from the group consisting oftitanium, zirconium and hafnium; said process comprising cleaning saidmetal surface by contact with a suitable cleaning bath, and thenconditioning said metal surface by contact with a pickling bath, whereinsaid pickling bath essentially comprises an aqueous solution ofhydrochloric acid and hydrofluoric acidand a soluble salt of iron,wherein one liter of said pickling bath contains the substantialequivalent of 900:10 cc. hydrochloric acid (18 B.) and 100:2 cc.hydrofluoric acid 70%.

7. The process of preparing the metal surface of a work-piece forplating thereon by a chemical plating bath 10 of the nickelcation-hypophosphite anion type, wherein said metal surface essentiallycomprises an amphoteric element selected from the group consisting oftitanium, zirconium and hafnium; said process comprising cleaning saidmetal surface by contact with a suitable cleaning bath, and thenconditioning said metal surface by contact with a pickling bath, whereinsaid pickling bath essentially comprises an aqueous solution ofhydrochloric acid and hydrofluoric acid and ferric chloride, theabsolute concentration of ferric chloride in said pickling 20 bath beingin'the general-range 0.09 to 0.36 mole/liter, the absolute concentrationof hydrochloric acid in said pickling bath being about 8 mole/liter, andthe absolute concentration of hydrofluoric acid in said pickling bathbeing about 4 /2 mole/liter.

8. The process of preparing the metal surface of a work-piece forplating thereon by a chemical plating bath of the nickelcation-hypophosphite anion type, wherein said metal surface essentiallycomprises a ti tanium alloy, said process comprising cleaning said metalsurface contact with a suitable cleaning bath, and then conditioningsaid metal surface by contact with a pickling bath, wherein saidpickling bath essentially comprises an aqueous solution of hydrochloricacid and hydrofluoric acid and a mild oxidizing agent, the absoluteconcentration of hydrochloric acid in said pickling bath being about 8mole/liter and the absolute concentration of hydrofluoric acid in saidpickling bath being about 4%. mole/liter.

9. The process of preparing the metal surface of a work-piece forplating thereon by a chemical plating bath of the nickelcation-hypophosphite anion type, wherein said metal surface essentiallycomprises a titanium alloy; said process comprising cleaning said metalsurface by contact with a suitable cleaning bath, and

then conditioning said metal surface by contact with a pickling bath,wherein said pickling bath essentially comprises an aqueous solution ofhydrochloric acid and bydrofluoric acid and ferric chloride, theabsolute concentration of ferric chloride in said pickling bath being in5 the general range 0.09 to 0.36 mole/liter the absolute concentrationof hydrochloric acid in said pickling bath being about 8 mole/ liter,and the absolute concentration of hydrofluoric acid in said picklingbath being about 4% mole/liter.

References Cited in the file of this patent UNITED STATES PATENTS2,717,218 Talmey et al. Sept. 6, 1955 2,774,688 Girard Dec. 18, 19562,825,682 Missel et al. Mar. 4, 1958 OTHER REFERENCES Brenner et al.:Part of Journal of Research of the Nation Bureau of Standards, ResearchPaper RH, 835, volume 39, pp. 385-395, November 1947.

r. MA.

UNITED STATES PATENT OFFICE I CERTIFICATE 0F CORRECTION Patent No, 2 928757 March 15, 1960 Warren G, Lee et a1,

It is hereby certified that error appears in the printed specificationof the above numbered patent requiring correction and that the saidLetters Patent should read as corrected below.

Column 8, line 3O after "surface" insert by =9 Signed and sealed this30th day of August 1960.

(SEAL) Attest:

ERNEST Wu SWIDER ROBERT C. WATSON W Attesting Oflicer Commissioner ofPatents

1. THE PROCESS OF PLATING WITH NICKEL THE METAL SURFACE OF A WORK-PIECE,WHEREIN SAID METAL SURFACE ESSENTIALLY COMPRISES AN AMPHOTERIC ELEMENTSELECTED FROM THE GROUP CONSISTING OF TITANIUM, ZIRCONIUM AND HAFNIUM,SAID PROCESS COMPRISING CLEANING SAID METAL SURFACE BY CONTACT WITH ASUITABLE CLEANING BATH, THEN CONDITIONING SAID METAL SURFACE BY CONTACTWITH A PICKLING BATH, WHEREIN SAID PICKLING BATH ESSENTIALLY COMPRISESAN AQUEOUS SOLUTION OF HYDROCHLORIC ACID AND HYDROFLUORIC ACID AND AMILD OXIDIZING AGENT, THE ABSOLUTE CONCENTRATION OF HYDROCHLORIC ACID INSAID PICKLING BATH BEING ABOUT 8 MOLE/LITER, THE ABSOLUTE CONCENTRATIONOF HYDROFLUORIC ACID IN SAID PICKLING BATH BEING ABOUT 4 1/2 MOLE/TIER,THEN CONTACTING SAID METAL SURFACE WITH A CHEMECIAL PLATING BATH OF THENICKEL CATION-HYPOPHOSPHITE ANION TYPE THROUGHOUT A SUFFICIENT TIMEINTERVAL TO OBTAIN A NICKEL PLATING UPON SAID METAL SURFACE, AND THENHEATING SAID WORK-PIECE TO A TEMPERATURE AT LEAST AS HIGH AS ABOUT425*C. SO AS TO EFFECT A DIFFUSION REACTION AT THE INTE